This invention is related to U.S. Pat. No. 4,557,180, patented Dec. 10, 1985.
The present invention relates to an electrohydraulic actuator and more particularly to a hydraulic system which provides matched flow rates for filling and emptying a variable volume load and provides positive blocking or holding of the load between changes.
As is understood by those skilled in the art, large process control valves, e.g. such as those employed in petroleum refineries and chemical and power plants are often driven by electrically controllable, hydraulic servo positioners. These devices are commonly referred to as electrohydraulic actuators. Such electrohydraulic actuators commonly employ a powerful hydraulic piston and a self-contained, positive displacement pump which provides a source of hydraulic power, both for stroking the piston and for holding same at any selected position within its stroke. Typically the pump is run continuously and the flow to the actuator is modulated by convential throttling means such as a flapper nozzle, spool valve or jet pipe system. The throttling means, in turn, is responsive to an electrical command signal through the use of force coils, torque motors or other electromagnetic devices, employed in conjunction with a position feedback loop.
As a result of the continuous pump operation, the efficiency of present state-of-the-art electro hydraulic actuators is, in the large majority of applications, in the order of five percent or less. Inherently a majority of the hydraulic energy generated by the pump is wasted as heat while the actuator is immobile. As is understood by those skilled in the art, the actuator is, in fact, immobile much of the time in most control valve applications, particularly in large and rather stable processes. Not only is the loss of energy wasteful, the heat created is itself troublesome. Typically, in an effort to minimize losses, the present state-of-the-art electro hydraulic actuators are operated under medium to low hydraulic pressure which necessitates cylinders having larger effective area and pumps having larger flow capacity, both factors being detrimental to cost of construction.
Among the several objects of the present invention may be noted the provision of electro hydraulic actuator in which the pump need be operated only when the piston is moving; the provision of such an actuator in which the piston position's volume is maintained by positive acting check valves when the piston is immobile; the provision of such a system which may be precisely controlled; the provision of such a system which has a symmetrical response; the provision of such a system which is highly efficient; the provision of such a system which is reliable and which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and, in part, pointed out hereinafter.